628 ORVILLE A. DERBY 



not so much, perhaps, in the areas occupied by the conglomerates 

 themselves as in the neighboring ones occupied by formations 

 known to have been in existence when the conglomerates were laid 

 down, and which have escaped being covered up by them or by later 

 strata. Until such prospecting is done on a sufficiently large and 

 efficient scale the opinion, which a few years ago seemed justified, 

 that a mode of occurrence essentially different from the African 

 must be postulated for these countries, should be held in suspense. 



The material filling the African pipes shows very pronounced 

 fragmenting and apparently explosive action, which has shattered, 

 and to some extent scattered, the eruptive rock characteristic of 

 both the pipes and the dikes and has mixed it with a very consider- 

 able amount of various other rocks, either brought up with it from 

 lower horizons or detached from the surrounding rock masses. Dis- 

 cussion is still going on as to whether the diamonds contained in 

 these agglomeritic pipe-fillings are to be assigned to the eruptive 

 rock proper or to some of the foreign rocks included in it, but the 

 weight of evidence seems to be in favor of the first hypothesis. A 

 very interesting view that was held for many years assigned the 

 formation of the diamond to some kind of reaction in situ, between 

 the two classes of rock that occur in the pipes, the necessary carbon 

 being supplied by the carbonaceous rocks through which, in some 

 places, the pipes cut. Subsequent developments have completely 

 disproved this hypothesis, but the essential part of it — the formation 

 of the diamond in situ — is still worthy of consideration if another 

 source of carbon can plausibly be brought into the question. 



So general is the association of the diamond with a fragmental 

 state of the eruptive rock that enters into the composition of the 

 pipes that the question naturally arises whether or not the diamond 

 also occurs in such masses of this rock as have not been subjected to 

 the fragmenting action. From the statements at hand it is clear 

 that there is usually considerable difficulty in distinguishing between 

 the massive and the fragmental forms of kimberlite. Apparently 

 the distinction has only been made in a perfectly conclusive manner 

 by the use of the microscope. The masses that can be thus exam- 

 ined are so small that such negative evidence as they may give has 

 in itself little value. Specimens of diamonds inclosed in fragmental 



